1. Field of the Invention
This invention relates to a bone-replacement prosthetic device, and in particular, to a method and apparatus for segmental bone replacement.
2. Description of the Related Art
In prosthetic segmental bone replacements, it is sometimes necessary to replace a portion of a long bone to correct various types of bone injury such as those caused by bone tumors, osteoarthritis, fracture dislocations, rheumatic arthritis, and aseptic or avascular bone necrosis. In these types of surgical procedures, it is necessary to resect a mid and/or end portion of a long bone and secure the remaining portion of bone through the use of some type of intramedullary device. This is accomplished by the use of one metal stem, or in certain cases, two opposing metal stems, secured in the medullary region by a "bone cement" or a grout material, such as methylmethacrylate.
Biologic fixation of intramedullary devices at the mid-diaphyseal level has not been entirely satisfactory, particularly in active younger patients, where it is important to form a stable, long-lasting prosthetic attachment. In time, the lack of adequate stress transfer from the metal stem to the surrounding bone causes a loss of bone density, resulting in increased possibility of bone failure or loosening of the bone-stem interface. Also, the bone reacts in the grout material or smooth metal stem by forming a soft-tissue lining around the cement, and this lining additionally mediates load transfer from the prosthetic device to the bone. The soft-tissue lining that forms about the device tends to loosen over time, particularly with continued shear loads, i.e., loads applied substantially in the direction of the axially extending bone/stem interface, and the loosening may become great enough in time to require surgical revision. Also, the relatively low tolerance of force transfer per unit area of interface requires a large bone/stem interface, which, in younger patients, may exceed the available interface area.
In replacement surgeries, which begin at the mid-diaphyseal level, the enlarged intramedullary cavities created within a remaining diaphyseal bone portion for the insertion and rigid fixation of a metal intramedullary device stem is also of small diameter. As a result, the device stems must be of a small diameter to fit within the diaphysis, with resulting poor rotational control. Also, high bending moments in thin stems at the mid-diaphysis risk fatigue failure, making a stable interface between the stem and the surrounding bone surface even more difficult to achieve.
Although the use of a tapered stem is one way to achieve increased security of the stem within the medullary canal, the canal dimensions tend to increase rather than decrease with increasing depth into the canal, thereby preventing secure wedging of a tapered stem. Consequently, "bone cement" or grout fixation is often used to secure the stem within the medullary canal. This technique, however, prevents stress transfer through the bone to the level of the osteotomy, therefore resulting in osteopenia adjacent to the stem. The same effect prevents bony ingrowth into porous pads on the shoulders of the implants. A limitation of prosthetic devices which rely on biological fixation, particularly fixation to an elongate stem within the intramedullary region of a bone, is the problem of stress protection of the bone region between the area of force application to the prosthesis and the area of load transfer to the bone. Stress protection is due to the rigid attachment between the prosthetic device and bone which occurs in biological fixation and to the relatively high elastic modulus of the implant material, which typically is five to fifteen times greater than that of the surrounding bone. These two factors combine to transfer a stress from the area of stress loading on the implant through the more rigid implant, rather than through the surrounding bone tissue. For example, in a hip-joint prosthesis biologically anchored to the bone by an entire elongate stem, axial stress on the upper joint is transferred largely through the stem to the bone connection farthest from the joint, rather than through the intermediate bone region surrounding the part of the stem closest to the joint. As a result, the intermediate bone region tends to be resorbed over time due to lack of deformation stressing. The gradual loss of bone support in the region of the stem increases the bending load that must be borne by the stem, and this can lead to implant fatigue and failure.
The problem of maintaining a motionless bone-prosthesis interface during the post-operative period when bony attachment is occurring may be partially solved by surgically fastening the prosthetic device to the bone structure by screws or the like. This method has been proposed for use in fastening a knee-joint prosthesis to a surgically formed, substantially planar surface of the bone. Typically, the prosthesis is attached by two or more screws, each tightened to hold the prosthesis against the bone surface with a selected compression. However, since the bone quickly accommodates to the applied force of the screws, by viscoelastic creep, the compression, and thus the resistance to the implant movement relative to the bone, is quickly lost. If interface movement does occur from a single episode of overloading, then any residual compression is permanently lost. More movements result in build-up of fibrous tissue, preempting biological bone fixation to the implant. Only with unphysiologic post-operative protection of the joint, resulting in joint stiffness and muscle wasting, and with demanding operative technique, can risk of loosening be reduced. The device also suffers from problems of stress protection and non-physiological load transfer, inasmuch as loading force applied to the prosthesis is transferred directly through the screws, rather than through the region of bone through which the screws extend. This can lead to loss of bone integrity in the stress protected area.
The problems associated with anchorage via soft tissue along a prosthesis stem have been overcome partially by using a prosthesis whose stem surface allows direct attachment without an interposed soft tissue layer. Such surfaces include micropore surfaces that allow attachment via ingrowth and/or attachment of bone, and ceramic surfaces that allow actual bonding of bone. Following surgical implantation of the stem, the surrounding bone tissue gradually forms a biological fixation matrix with the stem surface by tissue growth into or onto the surface. Because of the stronger interface between the bone and the stem, which allows a relatively large force per unit area without loosening, problems of late loosening and detachment are largely avoided and the force transfer area can be made smaller.
A limitation of the biological-fixation bonding approach, however, is the need to keep the prosthesis mechanically fixed with respect to the bone over a 2-3 month post-operative period, during which the biological fixation is occurring. If relative movement between the implant stem and bone is allowed to occur before biological fixation is complete, a fibrous tissue layer which acts to prevent good biological fixation develops at the interface and eventual progression to gross loosening is likely.
Another shortcoming resulting from bone replacement surgeries, particularly joint replacement procedures, is a phenomenon known as wear particle bone lysis. The replacement of a joint, including the installation of a polyethylene or similar high molecular weight synthetic wear surface, results over time in particles becoming dislodged from the wear surface due to friction between the joint sections during movement. These wear particles tend to move with fluid transfer along the interface between the prosthesis and the surrounding soft tissue, and also tend to enter the intramedullary space between the prosthesis stem and the surrounding remaining bone portion. The biological reaction to these small wear particles causes the surrounding bone tissue to be lysed, thereby weakening the bone and potentially causing subsequent bone failure. Thus, a means for sealing the intramedullary space from the exterior space could largely reduce this difficulty.
Prosthetic devices having spring-loaded mechanisms for holding a joint-replacement prosthesis against a planar surface of the bone, to immobilize the prosthesis on the bone, have been proposed, e.g., in the related field of joint replacements, such as in U.S. Pat. No. 4,129,903. Devices of this type solve some of the above-noted problems associated with prosthesis attachment to the bone, in that the prosthesis is held against the bone under relatively constant tension in the post-operative period, with or without provision for biological fixation. Nonetheless, limited movement may occur when the major loading stresses (in the principal direction of weight transfer on the joint) are not normal to the plane of the interface between the bone and prosthetic device and it is necessary to rely on a grouting compound to prevent shear motions. Further, such devices use a rigid stem or shaft for anchoring the implant to the bone traversed by the stem from physiologic shear, rocking, and/or axial rotation stresses.
Each of these potential problems may limit physical activity and long-term durability prognosis for long segmental replacement arthroplasty patients. Thus, current methods may potentially result in repeat surgeries which leave less bone stock and may eventually require amputation or other undesirable salvage procedures.
For the above reasons, it is desirable to provide a segmental bone replacement device which enhances a stable biologic fixation, yet allows for physiologic cyclic load transfer to the device-bone interface. It is also desirable to provide a device which promotes osteogenesis into those surfaces adjacent to the osteotomy.